Semi-closed and closed circuit breathing apparatus are used in a variety of hazardous professions such as deep-sea diving, fire fighting and hazardous material handling, just to name a few. In each application, the breathing apparatus must be designed to provide breathable gas in extreme environmental conditions that can vary significantly during times that the breathing apparatus is required. For example, a deep-sea diver's work capacity is severely limited by the physiological effects of high surrounding pressures and chilling seawater temperatures. Specifically, increased gas density has been shown to restrict the diver's ability to do useful work by limiting the maximum voluntarily ventilation in his lungs by up to 50% when in dry chamber environments at 1000 feet of seawater (FSW). The diver's ability to breathe using an underwater breathing apparatus (UBA) at elevated pressures is also restricted due to the inherent resistance of the UBA to the dense gas medium. Water temperatures as low as −2° C. necessitate reliable systems to protect divers from excessive heat losses through their clothing and during respiration.
An equally important concern to divers is the increasingly tight control that must be maintained on the quality and composition of their breathing gas as depth increases. The very gas that we depend on to sustain life on the surface becomes toxic to the deep-sea diver. For example, nitrogen in air becomes increasingly narcotic as depth increases, causing a rapid drop in performance and judgement. Air is generally replaced with a less narcotic helium-oxygen (heliox), or hydrogen-oxygen (hydrox), mixture when the partial pressure of nitrogen (PN2) exceeds 81.5 psi (5.55 atmospheres), equivalent to a PN2 when breathing air at depths beyond 190 FSW (˜58 meters).
In addition to the above-noted concerns, the nature of the human body must also be considered. For example, the human respiratory system contains an elegant set of defense mechanisms to protect the lungs from losing excessive heat when breathing cold, dry gases. The human respiratory tract with its intricate mucosal membrane filters foreign matter and bacteria from inhaled gases on their journey to the lungs. Additionally, the upper respiratory tract regulates the temperature and moisture content of the inhaled gases. In this way, the delicate gas exchange membranes in the lungs are protected from thermal injury and drying.
During inhalation, heat and moisture are added to the respiratory gases as they make their way from the nasal or oral passages to the alveoli. This heat is taken from a moving mucus blanket covering the upper respiratory tract (nose to the trachea). Past research has found that the temperature of inhaled air reaches 34° C. while its relative humidity reaches 80% before the air reaches the pharynx during respiration at surface conditions. By the time the air passes the trachea, the air generally reaches full body temperature and 100% relative humidity. During normal respiration of room air at 25° C. and 50% relative humidity, these heat and moisture demands on the nasal respiratory tract are relatively small, as they account for only 10–20% of the total body losses under resting conditions.
However, demand for heat and water vapor by a diver's airways increase substantially due to the effects of breathing dry, cold, dense gases at increased respiratory rates. At shallow depths, these heating demands are still relatively minor. Although drying of the airways due to gas humidification can be uncomfortable resulting in the notorious “cotton mouth” and dehydration during long dives. At depths greater than approximately 190 feet, helium makes up a large percentage of the respired gas. Helium, having a specific heat approximately five times that of air, requires a larger addition of heat to bring the inhaled gas up to body temperature. The combination of this high heat capacity and increased gas densities as the diver goes deeper results in respiratory heat losses for divers that are an appreciable part of the total body heat loss. This heat loss can even exceed the total metabolic heat production of the diver. Eventually, if unchecked, the diver's respiratory tract responds to these excessive heat demands with copious secretions that threaten the diver's life.
Obviously, it is apparent from the foregoing that breathing apparatus designs must be tested to determine their ability to address the various concerns of a particular application. Since it is not always practical or desirable to place personnel in dangerous conditions when testing a new breathing apparatus design, unmanned testing is used. Currently, such unmanned testing of closed circuit breathing apparatus involves the extraction of oxygen-rich breathing gas from the breathing apparatus and the replacement thereof with an inert gas. However, this approach does not provide test personnel with an understanding of how a human user would process the breathing gas as breathing apparatus conditions or environmental conditions are changed.